Nuclear reactor fuel designs, such as pressurized water reactor and boiling water reactor fuel designs, impose significantly increased demands nuclear fuel cladding tubes. Such components are conventionally fabricated from the zirconium-based metal alloys, such as zircaloy-2 and zircaloy-4. Increased demands on such components are in the form of longer required residence times, thinner structural members and increased power output per area, which cause corrosion. Resistance to radiation damage, such as dimensional change and metal embrittlement, is one of the most important considerations in selecting cladding materials for a fuel cladding tube. Zirconium alloys are currently used as the primary cladding material for nuclear fuel in nuclear power plants because of their low capture cross-section for thermal neutrons and good mechanical and corrosion resistance properties, high thermal conductivity and high melting point.
While zirconium and other metal alloys have excellent corrosion resistance and mechanical strength in a nuclear reactor environment under normal and accident conditions where the heat fluxes are relatively low, they encounter mechanical stability problems during conditions such as a departure from nucleate boiling (“DNB”) incident that might occur during accidental conditions. Any action tending to increase the heat flux of the core in order to raise the plant output will aggravate these problems. A significant in-reactor life limiting use with currently available fuel cladding tubes formed from zirconium-based alloys is corrosion, especially in the presence of water and increased operating temperatures of newer generations of nuclear reactors, such as light water reactors (LWRs) and supercritical water cooled reactors (SCWRs). For example, buildup of oxide material on the fuel cladding tubes caused by oxidation of zirconium during reactor operation may lead to adverse effects on thermal conduction. Hydrogen generated by oxidation of the zirconium in the fuel cladding tubes causes embrittlement of the zirconium and formation of precipitates in the fuel cladding tube which is under an internal gas pressure. The presence of the precipitates may reduce mechanical strength of the fuel cladding tube causing cracks in walls and end caps.
Fuel cladding tubes formed from zirconium and zirconium alloys are also susceptible to stress corrosion cracking during operation due to joint action of fission precuts and mechanical stress resulting from radiation-induced swelling of fuels. The interaction between the fuel and the cladding results in nucleation and propagation of cracks and depressurization of the fuel cladding tube. Such cracks propagate from an internal surface of the fuel cladding tube to an external surface and, thus, may rupture the cladding wall. Depressurization of the fuel cladding tube due to stress corrosion cracking significantly reduces the life of the fuel cladding tube and, in addition, reduces the output and safety of the nuclear reactor. Moreover, the fuel cladding tube may be circumferentially loaded in tension due to expansion of the contents, such as fuel pellets, within the fuel cladding tube. Deformation of the fuel cladding tube resulting from such tension increases susceptibility of the fuel cladding tube to stress corrosion failure.
Silicon carbide has also been used to form fuel cladding tubes for use in nuclear reactors. The silicon carbide fuel cladding tubes are porous structures that are difficult to achieve reliable hermetic sealed around the nuclear fuel. Thus, ceramic material debris dislodged from the silicon carbide fuel cladding tubes during use may reside between the fuel and the ceramic cladding and may be a source of local stress concentration during normal fuel swelling. The release of such ceramic material debris causes localized thermal and mechanical damage in the silicon carbide fuel cladding tubes, eventually causing failure thereof.
Given the resurgence of nuclear energy development worldwide there is a significant need today for both safety and economical performance enhancements to power plant or other reactor operations. Improved fuel cladding tubes that reduce operation costs and increase safety during reactor accidents are desirable.